Translation for `si` language

[markodvornik]

1. Add translation support for si language
2. Add messages for si

Possible solution: keycloakify/keycloakify#17 (comment)

Description

Lang support

Add locales=en,si configuration to the theme in a CI-frendly way.

Messages

As a long term solution, we should add translations for language si to the Keycloak project (keycloak theme messages).
As a project independet solution we should also implement translations in the templates (see keycloakify issue)